The present invention relates to a method and apparatus for detecting static fade in a digital video image such as a video frame. A method and apparatus for detecting static fade for individual macroblocks in a video image is also presented. In particular, the invention is suitable for detecting temporal fluctuations of ambient light intensity in a source video image. With reliable detection of a fade, coding efficiency can be improved.
Recently, digital video transmission formats have become increasingly popular for providing television and other audio, video, and/or data services to consumers' homes. Such programming can be transmitted to a consumer's home via a direct broadcast satellite link, cable television network, telephone network, or other broadcast means. In addition to providing new and expanded program offerings, digital video can provide a higher image and audio quality than traditional analog television broadcasts.
In order to transmit digital video signals within available bandwidths, data compression is necessary. In particular, spatial compression techniques take advantage of correlations between neighboring pixels or blocks of pixels in a frame, while temporal techniques take advantage of correlations between pixels or blocks of pixels in successive video frames. Moreover, motion compensation techniques can provide even greater temporal compression.
However, to provide optimal compression of a sequence of video frames, it is desirable to have the capability to detect fade in or fade out in the image sequence. Fade in refers to a gradual increase in light intensity over two or more frames or other images, while fade out refers to a gradual decrease in light intensity. Fades may be used by cinematographers and other video editors to provide a dramatic effect or to emphasize changes in a story line, for example.
Current motion estimation hardware in digital video encoders such as those conforming to the MPEG-2 standard utilizes block matching algorithms with the objective of minimizing the sum of absolute error in the prediction. However, this approach fails to cope with temporal variation of light intensity such as fade in and fade out in the source video. Details of the Motion Picture Expert's Group (MPEG) standard can be found in document ISO/IEC JTC1/SC29/WG11N0702, entitled "Information Technology--Generic Coding of Moving Pictures and Associated Audio, Recommendation H.262," Mar. 25, 1994.
It is desirable to detect a static fade, that is, a fade in or fade out effect on a still scene or on a scene that contains a small amount of motion, so that the video can be more efficiently coded. In particular, when a static fade is detected, the motion vectors can be forced to be zero, so less data is encoded and transmitted.
Various methods have been used to detect fade. For example, the sum of luminance pixel values may be computed for each frame. A scene is then classified as a fade if frame-to-frame changes in the sum exceeds a preset threshold. In another approach, the sum of luminance pixel differences between a current frame and the previous frame is computed. A scene is then classified as a fade if this sum exceeds a preset threshold. These methods are mathematically equivalent, although the implementation is different, and generally require the same amount of hardware. However, these approaches are generally not reliable for complicated scenes that contain a large amount of motion.
In another approach, a histogram of the pixel differences between the current frame and the previous frame is calculated. Fade is then detected by inspecting the histogram. This approach is more reliable, but the hardware required to implement a histogram calculator is relatively complex and expensive.
Accordingly, it would be desirable to have a method and apparatus for detecting fade in a video image sequence that is relatively simple to implement with low cost hardware, and requires only minor modifications to existing telecine or television detection hardware. The scheme should avoid false fade indications by distinguishing non-fade scenes such as stills and pictures that contain slow moving objects.
The scheme should further be compatible with existing digital video communication systems, including MPEG-1, MPEG-2, MPEG-4, ISO/IEC H.261 (videoconferencing), and ISO/IEC H.263. Moreover, the system should also be compatible with various color television broadcast standards such as the National Television Standards Committee (NTSC) standard, used in North America, and the Phase Alternating Line (PAL) standard, used in Europe, and should further be compatible with both frame and field mode video.
It would further be desirable to have a scheme for detecting static fade on sub-regions of a video image, such as on a macroblock by macroblock basis in a video frame, and setting the corresponding motion vectors to zero.
The present invention provides a system having the above and other advantages.